As is known, a radio receiver, in particular in a multimedia system of a motor vehicle, is able to receive a radio signal, in particular an FM radio signal, FM being the acronym of “frequency modulation”.
Such an FM radio signal, received in modulated form by a radio receiver, is subjected to various sensors and to suitable filtering so that the corresponding demodulated radio signal can be played under good conditions, especially in the passenger compartment of a motor vehicle.
Those skilled in the art know the operating principle of an FM, that is to say frequency-modulated, radio signal received by a suitable radio receiver, with a view to being demodulated and then played to listeners.
A known issue with the reception of an FM radio signal via a mobile radio receiver, in particular one integrated into a motor vehicle, is that of intermodulation.
Those skilled in the art know the principle of the effect referred to as intermodulation. In practice, the imperfection of the amplifier of the radio receiver in question induces a distortion of received radio signals. As is known, in particular, the characteristics of the distortion related to intermodulation may be defined in the following way: if two respective frequency signals F1 and F2 enter into a perfect amplifier, two superposed signals of frequency F1 and F2 are output.
Because of intermodulation, an imperfect amplifier outputs, in addition to the frequencies F1 and F2, other frequencies, namely F1−F2, F1+F2, 2×F1−F2, 2×F2−F1 and, generally, any frequency m×F1+n×F2, m and n being relative integers.
These frequencies m×F1+n×F2 are parasitic. Such intermodulation products may prove to be particularly disadvantageous when of 3rd order, when it is therefore a question of the frequencies 2×F1−F2 and 2×F2−F1, because of the associated powers remain high.
To mitigate this problem, a known solution consists in implementing a technique for automatically controlling gain, which is implemented before an element needing to be protected, such as a low-noise amplifier (LNA), a mixer or an analogue-digital converter. The principle of automatic gain control is known to those skilled in the art. The automatic gain control is implemented by a regulating circuit the function of which is to limit the amplitude of the signal that it outputs.
Automatic gain control, in the context of reception of FM radio signals, makes it possible to avoid, generally, any signal overload.
When intermodulation occurs and automatic gain control is active, the attenuation of all of the received signals in particular allows the intermodulation products to be more greatly attenuated.
Specifically, when the automatic gain control induces an attenuation of 1 dB in the first-order signal, the intermodulation products of the 3rd order are attenuated by 3 dB.
One major drawback of this prior-art technique is that very strong signals may create, because of intermodulation products, a phantom frequency substantially equal to a frequency of a desired but low-power radio signal. This creates co-channel interference.
In this context, implementation of automatic gain control may, in certain cases, induce an attenuation such that the desired weak radio signal is suppressed.
It will be noted, in this context, that the means for automatically controlling gain conventionally integrated into vehicle radio receivers may apply, to received FM radio signals, an attenuation of as much as −40 dB.
Another known technique for mitigating the problems created by intermodulation products consists in scanning the entire passband in order to identify all the strong signals and in calculating all the intermodulation products in order to identify potential “conflicts”. However, this technique requires a second tuner in the radio receiver and its implementation conventionally takes several tens of seconds, this being incompatible with the way in which users actually use radio receivers, and in particular radio receivers located on-board vehicles.